It's an amazing legacy. Ancient Greek drama flourished for less than a century, in just one city. Only a tiny fraction of the plays written have survived into modern times. And yet the influence they have had – not just on drama, but on every narrative art form, opera included – is immeasurable.

So what makes a tragedy? Writers from Nietzsche to Arthur Miller have rushed forward with their own definitions, but Aristotle, as one of the earliest writers on the form, remains for most the first port of call (though even he was writing a century or so after Greek tragedy's heyday). Still, the basics of his theory provide a valuable model.

Unity is central to Aristotle's definition of tragedy. The events of Greek dramas usually took place over just one day ('unity of time'). They often happen in just one location ('unity of place'). And they concern one protagonist, the events connected by direct, inexorable cause and effect ('unity of action').

That protagonist must be sympathetic. He or she should be generally decent, neither a saint nor a monster. It's only through an audience's ability to identify with the protagonist that the drama will be able to perform its dominant function; to bring about catharsis,by arousing pity and fear and ultimately providing resolution.

The events of the play lead towards the protagonist's catastrophe, or change of fortune. The catastrophe will be brought about both by some character flaw in the protagonist (what Aristotle called the hamartia) and also by the implacable action of cruel fate.

Simple plots depict just the catastrophe, but the best will include two further plot elements, tightly bound together through cause and effect. A character's actions have the opposite effect to the one he or she had anticipated in peripeteia, or reversal of intention. This leads to the protagonist'sanagnorisis, or moment of realization. This move from ignorance into knowledge precipitates the catastrophe.

For Aristotle, the play that came closest to perfecting the form was Sophocles' King Oedipus – the source for Mark-Anthony Turnage's opera Greek. Oedipus is our protagonist. His hamartia is that he does not know his true parentage. Oedipus is worried by prophecies that predict he will kill his father and sleep with his mother. The peripeteia is when a messenger, hoping to allay his fears, informs him that he is in fact not his parents’ son. His action actually prompts Oedipus to ask a series of questions that lead to his anagnorisis: he discovers he really has, in ignorance, killed his father Laius and married his mother Jocasta. This triggers the catastrophe: Jocasta hangs herself and Oedipus plunges the gold pins of her dress into his eyes, begging his brother-in-law/uncle to exile him.

There aren't many tragedies that follow Aristotle’s strict criteria to the letter. Turnage and Stephen Berkoff – who wrote the play that Turnage adapted for his libretto – have a last trick up their sleeve that subverts Sophocles' bleak moralistic message. But it's astonishing how many stories from across the intervening millennia map closely onto Aristotle's model, from Hamlet to The Wicker Man, from the Ringcycle to Mayerling. It's a formula for compelling, moving, excruciating drama – and probably will continue to be for as long as people are telling stories.